In 1940, during the Battle of France, four hundred thousand Allied soldiers have retreated to Dunkirk. Tommy, a young British private, is the sole survivor of a German ambush. At the beach, he finds thousands of troops awaiting evacuation and meets Gibson, who is burying a body. After a German dive-bomber attack, they find a wounded man. They rush his stretcher onto a hospital ship, hoping to remain aboard, but are ordered off. The ship is sunk by dive bombers; Tommy helps another soldier, Alex, out of the water. They leave at night on a destroyer, but it is sunk by a U-boat. Gibson saves Tommy and Alex from the sinking ship, and they get back to the beach. With only a single, vulnerable mole available for mooring deep-draft ships, the Royal Navy requisitions civilian vessels in the U.K. that can get to the beach. In Weymouth, a civilian sailor Dawson and his son Peter set out on his boat Moonstone, rather than let the Navy commandeer her. Impulsively, Peter's teenage friend George joins them. At sea, they rescue a shivering shell-shocked soldier from a wrecked ship. When he realizes that Dawson is sailing for Dunkirk, the soldier demands that they turn back and tries to wrest control of the boat. In the struggle, George falls and suffers a head injury that renders him blind. Three Spitfires cross the English Channel, heading towards Dunkirk.